This invention relates to voltage drop compensating devices for electrical power distribution systems. In particular, this invention relates to a solid state passive voltage drop compensating reactor for connection to a power supply.
In many electrical applications it is desirable to interpose an inductor between the power supply and the load. Systems distributing an electrical power supply through a building or industrial facility are often subjected to harmonic currents generated by non-linear loads such as electronic equipment (including computers, adjustable speed drives (ASD), uninterruptable power supplies (UPS), power rectifiers, etc.) and equipment that uses different kinds of arc processes (including arc discharge lighting systems). These harmonic-generating loads generate various levels of conventional harmonics (5th, 7th, 11th, 13th, 17th, 19th, 23rd, 25th etc.) and, for single phase line-to-neutral non-linear loads, also zero phase sequence or xe2x80x9ctriplenxe2x80x9d harmonics (3rd, 9th etc.) in the power distribution system, the harmonic spectrum depending upon the nature of the harmonic-generating load.
For example, FIG. 1 illustrates a typical current consumption waveform of a computer load at 60 Hz fundamental frequency, the accompanying table illustrating the distribution of harmonic currents present in the power distribution system as a percentage of the fundamental current. FIG. 2 illustrates a typical current consumption waveform of an ASD at 60 Hz, the accompanying graph illustrating the distribution of harmonic currents present in the power distribution system.
These harmonic currents create many problems in the power distribution system, including increased voltage total harmonic distortion level, reduced electromagnetic compatibility of the loads, reduced reliability of the power distribution equipment, increased power losses, reduced power factor, and other problems which are well known to those skilled in the art.
Prior art systems for mitigating harmonic currents fall into five basic types:
1. Power factor corrected (PFC) power supplies: In these systems the rectified current is continually adjusted to smooth the current consumption waveform. An example is illustrated in FIG. 3. PFC""s are relatively expensive devices and their applications are limited. Also, PFC""s cannot be retrofitted for use with existing power supplies, and are not practical for use with large ASD""s.
2. Active filters: These devices inject into the conductors between the power distribution system and the load, harmonic currents having a polarity opposite to those generated by the load, thereby neutralizing harmonic currents flowing into the power distribution system. An example is illustrated in FIG. 4. Active filters have many disadvantages, including high cost, poor reliability and poor dynamic characteristics. Active filters also are not practical for use with large ASD""s.
3. Resonant L-C filters: L-C filters are commonly used in power systems, tuned to different harmonic frequencies to mitigate specific harmonic currents. An example is illustrated in FIG. 5. These devices present many problems which are well known to those skilled in the art, including high cost, poor effectiveness in low voltage systems and the tendency to cause the system to operate with a leading power factor. Further, because L-C filters are non-directional they are easily overloaded by untreated harmonic currents generated by other harmonic sources connected to the power distribution system (for example in a neighboring facility), resulting in overloading and frequent failures of the filter""s capacitor bank.
4. AC chokes: In this harmonic mitigating technique reactors are connected in series between the line and the load. An example is illustrated in FIG. 6a (without a core) and 6b (with a core). This technique is simple, reliable and relatively low cost, however it results in a high voltage drop across the reactors. To reduce the voltage drop one must reduce the choke reactance level, which commensurately reduces the effectiveness of the choke and substantially limits harmonic current mitigation.
The voltage can be boosted by connecting a capacitor bank between the load and the choke, as shown in FIG. 7, but this frequently causes the system to operate with a leading power factor (especially in the case of light loading). In this case, since the reactance of the reactor at harmonic frequencies is much higher than the reactance of the reactor at the fundamental frequency, a large portion of the harmonic currents drain to the neutral through the capacitor. The capacitor has a high reactance at the fundamental frequency. However, the voltage drop across the choke remains very high. Thus, large compensating capacitors must be connected between the load and the choke to boost the voltage, which substantially increases the size and cost of the system and causes the system to operate at increased voltage levels during light loading conditions.
5. Phase shifting systems: Different kinds of phase shifters are available which allow the creation of quasi-multiphase systems, reducing certain harmonic levels. Harmonic currents of targeted orders are cancelled or substantially reduced depending upon the selected degree of the phase shift. However, such systems are typically limited in terms of the number of harmonic orders which can be mitigated, and the degree of harmonic mitigation depends upon the extent to which harmonics produced by the various harmonic sources are identical and their phase shift angles.
In other applications, an inductor may be used to smooth the dv/dt (acceleration) of a voltage (or current) waveform, for example between a motor and a variable speed drive controlling the motor; to reduce rf noise caused by some nonlinear loads such as arc welding apparatus; or to reduce sudden voltage drops caused by simultaneous activation of multiple loads (for example arc welding apparatus) sharing a common power supply. However, in each case where a series-connected reactor is used the voltage drop across the reactor remains high.
The present invention overcomes these disadvantages by providing a voltage drop compensating reactor connected between the power distribution system and the load which provides a flux shifting multiple-winding reactor with a capacitor bank connected between the reactor windings. At least one reactive element comprising a line winding connected to a first line, for example a phase, is oriented in a first polarity on a core. This provides a high reactance to higher frequency currents such as harmonic currents and rf noise generated by the load and flowing toward the power distribution system. According to the invention at least one compensating winding is provided on the same core, interposed in the first line between the line winding and the load, but oriented in a polarity opposite to the polarity of the line winding. The opposing fluxes generated by the line winding and the compensating winding cancel to a desired extent, and the cancellation of fluxes reduces the through-reactance of the device (i.e. supply-to-load) and thus reduces the voltage drop across the device between the power system and the load.
In a preferred embodiment an intermediate point in the first line between the line winding and the compensating winding is connected to another line, which may be a neutral or a different phase (in a multi-phase system), through a cross-link circuit comprising a capacitor. The capacitor provides a high reactance to the fundamental current, while at the same time creating a lower reactance path for higher frequency currents, such as harmonic currents and rf noise, between the load and the neutral (or different phase). As a result a large portion of the higher frequency currents flows through the cross-link, which offers a lower reactance at higher frequencies than the line winding, and thus penetration of higher frequency currents into the power distribution system is substantially mitigated.
The flux cancellation reduces the through-reactance of the reactor, i.e. the reactance of the path between the power distribution system and the load, which accordingly reduces the voltage drop across the reactor (since voltage drop is proportional to reactance). At the same time, the line winding introduces a high reactance level between the cross-link circuit and the power distribution system, so higher frequency currents such as harmonic currents are diverted through the cross-link circuit and are substantially eliminated from the system. This significantly decreases power losses in the power distribution system, which reduces energy usage, avoids overheating, decreases voltage distortion and improves the power factor.
However, because the voltage drop across the device is substantially reduced by the cancellation of fluxes between the oppositely-directed line winding and compensating winding, the device of the invention eliminates the need for large capacitors to boost the voltage at the load, and also substantially reduces voltage fluctuations resulting from load changes.
Moreover, the reactor and the capacitor bank operate over a range of frequencies, so in addition to reducing the voltage drop caused by a single inductor (or multiple inductors having the same polarity), the compensating reactor of the invention also effectively operates as a wide band filter so that a single device can be used to mitigate multiple harmonic frequencies and rf noise.
In the preferred embodiment the cross-link circuit also includes a winding disposed on the same core as the line winding, or optionally on a separate core, and connected in series with the capacitor in the same polarity as the line winding. This allows the frequency characteristics of the cross-link circuit to be selected as desired, providing an additional degree of freedom that allows the crosslink circuit reactance to be selected so as to more effectively divert harmonic currents of selected frequencies. Additional windings may be provided in the device of the invention, providing that in each case at least one compensating winding is provided between at least one line winding and the load, to cancel (partially or fully) the flux generated by the line winding and reduce the voltage drop across the device between the power supply and the load.
In a multi-phase system the compensating windings may be disposed on different core legs from the line windings to which they are respectively connected. This alters the reactance characteristics of the device, because the opposite fluxes generated by the line winding and its associated compensating winding will be out of phase, and at the same time changes the phase shift angle of the harmonic currents generated by a non-linear load. Also, in a multi-phase system the cross-link circuits of different phases can be interconnected to divert harmonic currents to a capacitor bank, with or without connection to a neutral.
These and other variations of the invention will become apparent from the description of the preferred embodiments which follows.
The present invention thus provides a voltage drop compensating reactor, comprising a magnetic core having at least one non-magnetic gap, a first reactive element comprising a line winding having a first end for connection to a line of the power distribution system and a second end, the line winding being disposed on the core in a first orientation, a second reactive element comprising a compensating winding having a first end for connection to the load and a second end, disposed on the core in a second orientation opposite to the first orientation, the second end of the compensating winding being connected to the line through the line winding, and at least one cross-link circuit comprising a capacitor having an input connected to the first line at an intermediate point between the line winding and the compensating winding, and an output connected to a second line of the power distribution system, wherein a voltage drop across the first reactive element is compensated by the second reactive element and any higher frequency currents generated by the load are diverted through the cross-link circuit..
The present invention further provides a voltage drop compensating reactor for use in a three phase power distribution system, comprising a magnetic core having at least three core legs and at least one non-magnetic gap, for at least one phase, a first reactive element comprising a line winding having a first end for connection to a first line of the power distribution system and a second end, the line winding being disposed on a leg of the core in a first orientation, a second reactive element comprising a compensating winding having a first end for connection to the load and a second end, disposed on the core in a second orientation opposite to the first orientation, the second end of the compensating winding being connected to the first line through the line winding, and at least one cross-link circuit comprising a capacitor having an input connected to the first line at an intermediate point between the line winding and the compensating winding, and an output connected to a second line of the power distribution system, wherein a voltage drop across the first reactive element is compensated by the second reactive element and any higher frequency currents generated by the load are diverted through the cross-link circuit.
The invention further provides a method of reducing a voltage drop across a line reactor in a first line of a power distribution system, the line reactor comprising a line winding disposed on a magnetic core having at least one non-magnetic gap, the line winding having a first end connected to the first line and a second end connected to the load, comprising the steps of a) connecting a compensating winding in series between the second end of the line winding and the load, the compensating winding being disposed on the core and oriented in a polarity opposite to the polarity of the line winding to thereby generate a flux in a direction opposite to a flux generated by the line winding, b) connecting the input of a cross-link circuit comprising a capacitor to the first line at an intermediate point between the line winding and the compensating winding, and c) connecting the output of the cross-link circuit to a second line, whereby an impedance between the first end of the line winding and the load is lower than an impedance between the first end of the line winding and the second end of the line winding.
The invention further provides a voltage drop compensating reactor for interposition between a load and a multi-phase power distribution system, comprising a plurality of magnetic cores, each having at least one non-magnetic gap, a plurality of line windings each having a first end for connection to a first line and a second end, each line winding being disposed on a separate one of the plurality of cores in a first orientation, a plurality of compensating windings each having a first end for connection to the load and a second end, each compensating winding being disposed on one of the plurality of cores in a second orientation opposite to the first orientation, the second end of each compensating winding being connected to a line winding, and at least one cross-link circuit comprising a capacitor having an input connected to an intermediate point between at least one line winding and a compensating winding connected to the at least one line winding, and an output connected to a second line, wherein a voltage drop across the first reactive element is compensated by the second reactive element and any higher frequency currents generated by the load are diverted through the cross-link circuit.
Various additional line windings, compensating windings, cross-link circuits and/or cross-link windings may be provided or omitted as desired to improve the ability to target specific harmonic or other higher frequency currents.